Add the cubed chicken tenders to a medium mixing bowl (or large Ziploc bag) along with the soy sauce, dry sherry, vegetable oil, and cornstarch. Toss to combine until the chicken is evenly coated. Let that sit at room temperature for 15-20 minutes.
Prep the sauce by whisking together the water, soy sauce, rice wine vinegar, sesame oil, hoisin sauce, sugar, cornstarch, and red pepper flakes. Ensure the cornstarch and sugar are fully dissolved, and then set aside.
Place a large skillet over medium heat. Add 1 tablespoon of oil and cook the bell peppers for about 5 minutes, stirring occasionally. Transfer to a bowl and set aside.
Add some oil (1 tablespoon at a time) to the skillet and work in batches to brown the chicken. Cook for a few minutes on each side until the chicken is cooked through and no longer pink. Pro Tip: It usually takes me 4 batches to cook this amount of chicken and get a good sear. Keep an eye on the heat and reduce the burner temp as needed so the chicken doesn't burn.
Reduce the heat to medium and add all the chicken back to the skillet along with the bell peppers, garlic, and ginger. Cook for about 1 minute, until the garlic and ginger are fragrant.
Pour the kung pao sauce into the pan and cook until the sauce has thickened.
Stir in the peanuts and green onions. Serve warm over rice. Garnish with more green onions if desired.